Isotope ratio mass spectrometry is a technique which accurately and precisely measures variations in the relative abundances of isotopes, i.e. isotopic ratios, of elements such as 13C/12C, 18O/16O, 16N/14N and 34S/32S in molecules.
Prior to analysis, a sample typically undergoes oxidation, pyrolysis or reduction at an elevated temperature to produce gases of molecules, for example, COx, NOx, H2O. The gases are then introduced into the IRS for isotopic analysis. In the isotope ratio spectrometer (IRS), the gases are ionised and the ratios of corresponding isotopes are measured for example by comparing outputs of different collectors. The ratios of the isotopes of interest are typically measured relative to an isotopic standard in order to eliminate any bias or systematic error in the measurements.
For isotopic analysis of specific compounds within a complex mixture, it is desirable to perform a separation prior to the isotopic analysis. Currently, this separation is performed by gas chromatography, which can be coupled to an IRMS using a combustion oven.
Liquid chromatography (LC) is an established technique in the field of biochemistry and pharmacology. However, coupling an IRS to a liquid chromatography system presents technical challenges because LC mobile phase is usually organic and therefore produces the same products as sample molecules of interest, thus interfering with the isotopic analysis. There have been various attempts at coupling liquid chromatography to IRMS, as identified below.
“Moving-wire device for Carbon Isotopic Analyses of Nanogram Quantities of Nonvolatile Organic Carbon” (A. L. Sessions, S. P. Sylva and J. M. Hayes, Anal. Chem., 2005, 77, 6519-6527) describes a method for analysing 13C ratios of involatile organic samples dissolved in solution. The output solution of the separation system is dried onto a nickel wire to remove the mobile phase from the sample. The residual sample is then combusted and the evolved CO2 is analysed by IRMS. However, both the precision and sensitivity of this method are limited by a high background level of CO2 derived from carbon within the wire.
Another method of coupling a liquid chromatography system to an IRMS is presented in ““Continuous-Flow Isotope Ratio Mass Spectrometry Using the Chemical Reaction Interface with Either Gas or Liquid Chromatography Introduction” (Y. Teffera, J. Kusmierz, F. Abramson, Anal. Chem., 1996, 68, 1888-1894)”. In this method, the solution exiting from the liquid chromatography system undergoes desolvation at semi-permeable membranes prior to chemical oxidation of the dry aerosol. The oxidised products are then analysed by IRMS. However, the method described does not remove the mobile phase to the required ultra-low levels of solvent, for example, to a solvent/sample ratio better than 1:100.
Wet chemical oxidation (LC-Isolink™) addresses the problem of both earlier methods and allows coupling to liquid chromatography. The solution output from the chromatography system is mixed with an oxidizing agent and supplied to an oxidation reactor. In the oxidation reactor the organic compounds are converted into CO2, which is then analysed in the IRMS. However, there is no separation of the mobile phase from the sample and therefore, this method is not suitable for separation methods that require an organic mobile phase.
In the fields of pharmaceutical and life sciences, the typical sample includes organic molecules dissolved in an organic solvent. For such samples, separation of the molecules from the solvent is generally carried out with an organic mobile phase using techniques such as high performance liquid chromatography, capillary-zone electrophoresis and size-exclusion chromatography. As a result, the output of the separation apparatus also consists of an organic sample dissolved in an organic solvent.
The presence of this organic solvent would result in production of a large amount of CO2 during combustion and hence an extremely high background CO2 in the spectrum produced by IRMS.
In order for analysis by IRMS of an organic molecule dissolved in an organic solvent, a great reduction in the solvent/sample ratio from 100-1,000,000:1 to less than 1:100-1000 i.e. a reduction of 5-8 orders of magnitude or higher is required.
None of the existing techniques, as identified above, can reduce the organic solvent/organic sample ratio to the required ultra-low levels.
Therefore, a sample introduction system which can couple a supply of sample entrained with any solvent to an IRMS is required.
The present invention seeks to address this problem by providing a new approach to separation of sample molecules from more volatile molecules of the mobile phase.